VCSELs are widely used as light sources for optical interconnect devices, storage area networks, and sensors. The most common configuration of a VCSEL is a two-terminal VCSEL that includes a conducting n-type substrate, an n-type distributed Brag reflector (DBR) disposed on the top surface of the substrate, an intrinsic layer (active region) disposed on top of n-type DBR, a p-type DBR disposed on top of the intrinsic layer, an ohmic n-contact, and an ohmic p-contact. The ohmic n- and p-contacts correspond to respective first and second terminals of the VCSEL. When an electric potential is applied across the terminals, electrons from the n-type layers that are adjacent the intrinsic layer and holes from the p-type layers that are adjacent the intrinsic layer are injected into the active region of the intrinsic layer where they combine to produce photons. This combining of holes and electrons in the active region to produce photons is a phenomenon known as spontaneous emission. As the photons pass out of the active region, they are repeatedly reflected by the DBRs back into the active region, which results in more recombination of electrons and holes in the active region. This is a phenomenon known as stimulated emission. The repeated reflection of photons by the DBRs back into the active region provides the “pumping” action that leads to lasing.
In the two-terminal VCSEL described above, the configuration of the intrinsic layer sandwiched between the n-type and p-type DBRs forms a p-i-n junction, i.e., a diode. The application of the electric potential across the terminals forward biases the junction to cause the electrons and holes to be injected into the active region. The modulation speed of the VCSEL is limited by the interaction between the electrical carriers (electrons and holes) and the photons in the active region.
Three-terminal VCSELs are also known, although they are less common than two-terminal VCSELs. In the two-terminal VCSEL, the p-i-n junction is essentially a base-emitter junction. In the three-terminal VCSEL, a collector is added to give the VCSEL a bipolar junction transistor (BJT) configuration having a base sandwiched between the emitter and the collector. In such configurations, an electric potential is applied across first and second ohmic contacts connected to the base and emitter, respectively, to forward bias the junction, which results in current injection, and ultimately, lasing. An electric modulation signal is also applied across these ohmic contacts to modulate the laser, i.e., to turn it on and off. In some cases, an electric modulation signal is also applied across a third ohmic contact connected to the collector and the first ohmic contact to assist in modulating the active region.
One of the problems associated with the three-terminal VCSEL configuration described above is that the addition of the collector does not increase the speed of the VCSEL. The speed of the VCSEL continues to be limited by the interaction between the carriers and photons in the active region.
Another configuration of a three-terminal VCSEL is disclosed in U.S. Pat. No. 7,693,195. In this configuration, an electrical modulation signal can be applied to the collector terminal and output from the collector terminal. An optical signal is output from the base region. A forward-biasing electrical modulation signal is applied to the base and emitter terminals while a constant reverse-biasing electrical signal is applied to the collector terminal relative to the emitter terminal. Thus, the emitter-collector junction is constantly reverse-biased while the base-emitter junction is switched between a high forward-biased state and a low forward-biased state to switch the laser between a logical HIGH state and a logical LOW state of an input signal, respectively. The input modulation signal applied to the emitter-base junction produces a light output as well as an amplified electrical signal on the collector terminal. One of the problems associated with the design, however, is that, in order to produce an amplified output electrical modulation signal at the collector terminal, the base region must be designed to be thin so that the majority of electrons from the emitter are able to pass across the base region and arrive at the collector, resulting in fewer electrons available for recombining with holes in the base region to produce light. Consequently, the optical output at the logical HIGH state of the laser is lower than the optical output at the logical HIGH state in typical two-terminal VCSELs.
Accordingly, a need exists for a three-terminal VCSEL that is capable of achieving higher speeds than known two-terminal and three-terminal VCSELs without sacrificing optical output.